To prepare silicon nitride powders on industrial scale, various methods of direct synthesis have found wide application since initial materials are readily available.
Known methods of direct synthesis of silicon nitride powder via the nitration reaction can be divided into traditional methods of furnace synthesis widely used in industry and a promising relatively new plasmochemical method and a method of self-propagating high-temperature synthesis (SHS) in the combustion regime.
The method of furnace synthesis of silicon nitride powders is based on nitration of elemental silicon powder upon heating in electric furnaces in a flow of nitrogen or nitrogen-containing gas. Nitration of silicon dioxide in a mixture with a reducer, mainly, carbon, is one of the versions of the known method.
Methods of silicon nitration in furnaces are predominantly two-stage and their accomplishment requires much time. The first stage of silicon powder nitration is carried out at a temperature by 100.degree.-250.degree. C. below the melting point of silicon and up to the attainment of 30-40% degree of bonding silicon to nitrogen. This stage requires from 3-5 hrs to 10-20 hrs. At the second stage complete nitration is performed at 1500.degree.-1600.degree. C.
Known in the art is the most effective and rather simple method of preparing silicon nitride powder with a high content of .alpha.-phase by the furnace synthesis method. The method resides in nitration of metallic silicon in a flow of a nitrogen-containing gas under a reduced nitrogen partial pressure for 4-5 hrs upon heating in furnaces at 1200.degree.-1400.degree. C., the nitrogen partial pressure being maintained equal to .about.0.5 atm (0.05 MPa) as long as 50-60% by mass of silicon is reacted. The method provides the content of .alpha.-phase in the silicon nitride powder equal to 97% by mass.
The use of the known method for preparing silicon nitride requires a strict control over temperature, nitrogen partial pressure, and a gas flow rate in the course of the whole nitration process in order to maintain the thermal conditions required for the formation of .alpha.-phase and to compensate the exothermal effect of the nitration reaction with the aid of heat removal. Besides, this method demands rather great power consumption for attaining 1200.degree.-1400.degree. C. in the electric furnaces at which the nitration process takes place.
The method of plasmochemical synthesis, namely, nitration of silicon in a low-temperature nitrogen plasma, is of interest for preparing ultrafine silicon nitride powders possessing good caking ability.
Known in the art is a plasmochemical method of preparing silicon nitride by nitration of silicon in a nitrogen plasma produced by a high-frequency generator with the use of 98.9% pure silicon and nitrogen of a high purity (Izd. Akad. Nauk SSSR, ser. Neorganocheskie materialy, Moscow, 1979, Vol. 15, No.4. G. M. Kheidemans, Ya. P.Grabas and T. A. Miller "High-temperature synthesis of finely dispersed silicon nitride", pp. 595-598).
Silicon nitride powder prepared by the above method is a mixture of .alpha. and .beta. phases and contains 2-4% by mass of free silicon and up to 5% by mass of oxygen. In addition, plasmochemical powders of silicon nitride possess an enhanced chemical activity as compared with powders obtained by other methods and are readily hydrolyzed in humid air which requires certain measures upon storage and processing.
Thus, although plasmochemical powders possess a tendency to cake, the quality of these powders does not allow one to use them for preparing ceramic materials with high physical and mechanical properties.
Besides, the accomplishment of the plasmochemical synthesis, as well as that of the furnace synthesis, demands great power consumption.
The method of self-propagating high-temperature synthesis (in the combustion regime) (U.S. Pat. A, No. 3726643) is the most promising one for direct synthesis of silicon nitride with respect to the purity and quality of the prepared product, efficiency and energy-intensity of the process.
The method is based on the use of heat liberated upon exothermal interaction of the reagents at least one of which is in a condensed state. The method resides in local initiation of a chemical reaction in a layer of the reaction mixture and in a subsequent interaction of the reagents in the combustion regime, i.e. self-propagation of the combustion front at the expense of layer-by-layer self-heating of the reaction mixture due to sufficient exothermal effect of the reaction.
A great thermal effect of the reaction of silicon with nitrogen (180 kcal/mole) makes it possible to carry out the process of silicon nitration in the combustion mode, i.e. by the method of self-propagating high-temperature synthesis (SHS).
Moreover, the exothermic effect of the reaction of silicon with nitrogen is so high that the combustion temperature must be decreased in order to maintain the temperature preferable for the formation of .alpha.-phase of silicon nitride.
To decrease the combustion temperature, the initial powderous mixture of the reagents is diluted to 50% by mass with the final product.
Silicon nitride with a high content of .alpha.-phase was prepared by the method of self-propagating high-temperature synthesis with dilution of the initial powderous mixture by the final product (J.Am. Ceram. Soc., 1986, Vol. 69, No. 4; Kiyoshi Hirao, Yoshinary Miyamoto, Mitsue Koizume "Synthesis of silicon nitride by a combustion reaction under high nitrogen pressure", pp. 60-61).
The method resides in preparing an initial powderous mixture (charge) from silicon powder (99.9% pure) with a dispersity of .ltorsim.5 mkm and silicon .alpha.-nitride powder (98% of .alpha.-Si.sub.3 N.sub.4) with a dispersity of .about.0.1 mkm at a mass ratio of 47.4 and 52.6% respectively, grinding the charge components to a dispersity of .about.0.2 mkm, mixing in acetone, drying the charge in a vacuum, molding the cylinders 6 mm in diameter and 10 mm long with a density of 44-46% of theory, and performing the synthesis in the combustion mode under 10 MPa after local initiation of the reaction in a charge layer with a 3 s electric pulse.
The synthesis in the combustion mode yielded silicon nitride with 87% by mass of .alpha.-phase.
Thus, the preparation of silicon nitride with a high content of .alpha.-phase requires the use of silicon nitride with almost a 100% content of .alpha.-phase in amount of 50% of the initial charge mass as an inert component in making the initial charge.